1. Field of the Invention
The present invention relates to an exposure system and an exposure method for forming a semiconductor device, and more particularly to a method and a system for controlling an exposure value in an exposure system and an exposure method to reduce variation of a resist pattern size.
2. Description of the Related Art
A lithography process is used for forming a semiconductor device. A resist film is applied by an applicator onto a semiconductor wafer surface, and the resist film is then baked, before an exposure and a subsequent development are carried out. After the applied resist film is previously baked, it is actually needed to take a waiting time for starting the subsequent exposure process. In this application, the word xe2x80x9cWaiting timexe2x80x9d is defined to be a time duration after a pre-baking process for the applied resist film and until an exposure process is started. For this waiting time, the wafer is stoked between the applicator and the exposure system. This waiting time is variable for individual carrier units for matching the timings among the sequential processes.
It has been know that variation in waiting time of the wafer causes variation in size or dimension of the resist pattern after the exposure and development processes. FIG. 1 is a diagram illustrative of various sizes of a photo-resist pattern versus waiting time. The size of the photo-resist pattern is variable. As the waiting time increases, an averaged size of the photo-resist pattern tends to increase. As the waiting time increases up to a time T1 from zero, the rate of increase in the averaged size of the photo-resist pattern is rapid. As the waiting time further increases from the time T1 and approaches a longer time T5, the rate of increase in the averaged A size of the photo-resist pattern becomes gentle. As the waiting time becomes much longer, the variation in size of the photo-resist pattern becomes small.
As may be seen from FIG. 1, the variation in waiting time of the wafer causes variation in size or dimension of the resist pattern after the exposure and development processes. The waiting time may be different between different carrier units of the wafers. Thus, the size or dimension of the resist pattern may be different between different carrier units of the wafers.
An excessive increase of the waiting time is effective in order to reduce the variation in size or dimension of the resist pattern, but also reduces the throughput of the wafer. For this reason, the excessive increase of the waiting time is undesirable practically.
It has also been known that the variation in size of the resist pattern may depend on not only variation of the waiting time but also variation in the atmospheric pressure. If the atmospheric pressure is reduced, then the thickness of the resist is increased. The variation in the atmospheric pressure causes variation in the atmospheric density, which further varies the refractive index of an optical lens to an air, resulting in variation in the focusing point.
FIG. 2 is a schematic block diagram of a conventional semiconductor manufacturing system for photo-lithography processes and subsequent anisotropic etching process. A system 201 includes a host 210, a photo-resist applicator 202, an exposure and development apparatus 203, a first size-measuring device 205, an etching apparatus 204, and a second size-measuring device 206. The first size-measuring device 205 measures the size of the photo-resist pattern immediately after the development process. The second size-measuring device 206 measures the size of the etched region of the wafer immediately after the etching process.
The size-measured results by the first and second size-measuring devices 205 and 206 are transmitted to the host 210. The host 210 changes, if any, resist application conditions for applying the resist film by the applicator 202 and also exposure conditions by the exposure and development apparatus 203, and further etching conditions by the etching apparatus 204.
The above conventional technique depends upon the past-measured pattern sizes manufactured in the past process, in order to set the conditions for the future wafers, namely, not responsible in real time to the variations in the waiting time and the atmospheric pressure. It is, therefore, difficult for the conventional technique to suppress the size variation of the wafer based on the variations in the waiting time and the atmospheric pressure.
Japanese laid-open patent publication No. 8-172046 discloses that various data about a pre-baking termination time, wafer not numbers, and the kinds of the wafer resist are transmitted from a storage device through a data transmitter to an exposure controller, so that the exposure controller calculates the waiting time of the wafers based on the transmitted data in order to set an appropriate exposure value based on the calculated waiting time. This conventional technique avoids that the size of the resist patterns varies depending on the waiting time.
The above conventional technique of the above Japanese publication also depends upon the past-measured pattern sizes manufactured in the past process, in order to set the conditions for the future wafers, namely, not responsible in real time to the variations in the waiting time and the atmospheric pressure. It is, therefore, difficult for the conventional technique to suppress the size variation of the wafer based on the variations in the waiting time and the atmospheric pressure.
Consequently, the above two conventional techniques determine the exposure value for the resist film based on the past waiting time of the past-processed wafers in the past processes, without considering the current waiting time of the current wafer to be processed from now.
In the above circumstances, the development of a novel exposure system and exposure method free from the above problems is desirable.
Accordingly, it is an object of the present invention to provide a novel exposure system free from the above problems.
It is a further object of the present invention to provide a novel method of determining an exposure value free from the above problems.
The present invention provides a method of determining an exposure value for exposing a resist film, comprising the steps of: estimating a size variation of a resist pattern from a predetermined target size based on a waiting time of a currently processing resist film to be patterned in subsequent sequential exposure and development processes; and compensating a reference exposure value based on said size variation to obtain a compensated exposure value.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.